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  • please educate me on fork +ve / -ve pressure
  • Premier Icon pembo6
    Full Member

    I’ve tried googling, but there seems to be a lot of misunderstanding or misinformation on the interwebs about this.

    Does too much or too little negative pressure cause fork suckdown?
    I beleive it is too little pressure (or even a vacuum) that will cause the fork to suck down in its travel?? But there are nummerous forum posts that state too much negative pressure will cuase suckdown and people should cabletie burp their lowers at the dust seals to release some of the preaaure in the negative side.

    I understand the concept of how it generally happens. Air not transferring between positive and negative sides of the air spring.

    Having some issues with my current fork, and this might help diagnose whats actually happening. Or at least help me understand the physics of it all.

    TIA

    Premier Icon pembo6
    Full Member

    double post – deleted

    Premier Icon Onzadog
    Free Member

    Sounds like you might be confusing negative chamber and fork lowers.

    If you have a positive and negative chamber in your air spring, they can either be self setting (like a Rockshox Debonair equipped fork) or independent (like the older dual air Rockshox).

    If it’s self setting, you can’t adjust it. However, a dual air fork allows you to set the negative pressure through a separate valve. On this fork, more pressure will make the initial stroke more supple and could cause the fork to suck down.

    It’s not just a case of making the two pressures the same. The positive pressure acts on the top of the piston on the full area. The negative acts on the bottom of the piston but remember, there’s also an air spring rod on this side so the area is reduced.

    Both the positive and negative air spring are contained in the stanchion on the air spring side.

    There is air inside the lowers at atmospheric pressure when the forks are assembled. It’s not typically considered as part of the forks spring system, more just a consequence of being assembled by people on earth.

    When the fork compresses, this air increases in pressure and may burp out of the fork seal. When the fork extends, this pressure is now less than one atmosphere and therefore atmospheric pressure outside the fork will compress the fork until that pressure balances with the outside world or you slip a ziptie down the side of the seal.

    Premier Icon nickjb
    Free Member

    Positive holds it up, negative pushes it down

    At the top of the stroke they are pretty even so the fork is supple. Further into the stroke the positive increases, the negative has less effect

    Too much negative (ie more than positive) and the fork will go down on its own and stay there. Not enough negative and it’ll want to stay up, ie not be supple

    Premier Icon thols2
    Free Member

    Does too much or too little negative pressure cause fork suckdown?

    Too much negative pressure would cause the fork to suckdown.

    Older Rockshox forks had separate positive and negative chambers that you could inflate to different pressures. The positive spring is the main one that holds you up off the ground. You could set this so that you didn’t bottom the forks out, then adjust the negative spring to give a supple ride over small stuff.

    I guess about 10 years ago, they changed to the Soloair design. It still has a negative spring, but it’s automatically set to the same pressure as the positive spring. Generally, you’d set them the same anyway so this just made it easier for most people to set up the forks. I actually liked the DualAir system, but I’m guessing that most people prefer just setting one spring.

    As above, you may be confusing it with air leaking into the lowers. That’s a totally different thing.

    Premier Icon nickjb
    Free Member

    I’ll share my own “stuck down” story if it helps.

    My 2014(ish) lyriks kept getting stuck down. They could be burped by pulling them up, be fine for a while, then get stuck again. These forks have a valve that is activated when the fork tops out and equalises the pressure between the pos and neg chambers. This self balances the pressure at the top of the stroke. Turned out the top out bumper was loose. It would bounce around inside and sometimes trigger the valve further into the stroke so the negative chamber pressure was significantly increased. That higher negative pressure would keep the forks down

    Premier Icon pembo6
    Full Member

    Thanks for all the responses. That makes it clear.
    And yes, I was confusing negative volume and fork lowers.

    Premier Icon peaslaker
    Free Member

    This rabbit hole eventually leads to the “size” of the negative chamber – a marketing term that is ascribed mystical powers to simultaneously unleash hitherto unachievable equivalence to a spiral of springy wire while having a just rightness of progression (which rhymes with aggression) to tame the savage riding beast that lurks within us all.

    Premier Icon greyspoke
    Free Member

    Its more than a marketing term. My old Marzocchi Marathons had a tiny negative air chamber (and resulting high negative air pressure required). Bloody impossible to get the things right, either clangy top-out or stuck in the middle of the travel. It frustrated me so much I modelled the behaviour of a dual air spring with a bit of Boyle’s law. As I recall, the relative volumes of the two chambers is a key factor in what shapes of spring rate curve you can achieve. (But I think the problem with the Mazzas was the impossibility of setting the pressure sufficiently accurately, a tiny leak on disconnecting the valve and the whole thing would go to cock.)

    Manufacturers seem to have sorted this out with automatic pressure balancing so we don’t have to faff setting negative pressures any more, thank goodness.

    Premier Icon thols2
    Free Member

    the relative volumes of the two chambers is a key factor in what shapes of spring rate curve you can achieve

    Exactly. It’s not purely a marketing thing, even if the marketing people overhype it.

    Premier Icon peaslaker
    Free Member

    You two are no fun. But do you refer to bigger end-space or a deeper set equalisation port?

    See how much easier it is to refer to something in regard to its bigness. See all the potential for telling us something is bigger (better, obvs) while actually the engineers have just anodised the piece in a new colour and the journos can recycle the press release for this year’s widget that you’re selling for £24 a pop.

    Bigger = good. Right. So lets add (bigly) tokens to our big airchamber (+ve) to make it (let me see) small yet ascribe goodness because we’ve done a big/big combo. Muchly bigness. Progression is for real men (obvs).

    Premier Icon zippykona
    Full Member

    What is the correct way to set up a dual air fork? How would I improve small bump sensitivity?

    Premier Icon wordnumb
    Free Member

    Dual air fork = set the sag using just the +ve valve, once happy with sag add exact same pressure to -ve. Small changes to the -ve pressure may fine tune the fork sensitivity depending what you want.

    Personally I prefer the old dual air version to the newer solo air, yet I only ever use an equal pressure between the two valves, which should give the same result as the solo air. Madness.

    Premier Icon Northwind
    Full Member

    zippykona
    Full Member

    What is the correct way to set up a dual air fork? How would I improve small bump sensitivity?

    Experimentation tbh. My experience though was that I always got the best results by basically setting the negative to the exact point it started sucking the fork down. Just the tiniest bit. Reasonably enough when you think about it, small bump resistance is from the fork resisting the terrain, so that first hair of movement is the negative spring overcoming that same resistance.

    That’s a bit fiddly unfortunately because it means you can’t just set the positive to the sag point you want- you have to go a little higher, otherwise it sits too low when you’re actually riding. You can’t really figure it out in the garage, it only really works when it’s working.

    This sort of thing is why they stopped doing dual air- people found it hard to set up/understand. I loved it, solo air is such a massive compromise but if even a quarter of riders weren’t getting a good result then that’s a bigger problem. With solo air, everyone can get at least a useful setting, at the expense of those of us that like playing with stuff who now can’t get as good a setup as we could with dual.

    Premier Icon thols2
    Free Member

    With Dual Air, I always set the positive until I could get it working ok at the end of the stroke (using most of the travel, but not bottoming out, not too soft in the mid stroke), then added negative until it just barely began to suck down so it was compliant over small stuff.

    Premier Icon peaslaker
    Free Member

    For dual air, back in 2010-2011 I built a transfer valve with pneumatic piping to join both air chambers to the same pump and the same pressure. You could then turn a valve and separate the two before detaching the valves and sealing the system. This then becomes a very accurate system. You could equalise the two and then once separated you could add a pump stroke or two to one chamber or the other.

    The other variable is to do with where the fork positions itself as you’re setting up the pressures. If the fork is in its travel and not resting up against the top out stop, you don’t really have predictability. As soon as you start seeing any sort of suck down from applying negative pressure you’re basically in a place where you have very little repeatability. All the advice to set positive first and then set negative assures that the fork is against the top out as you’re setting positive – this is good advice. The alternative approach (with my transfer valve) was to set the pressure and then compress the fork slightly (5-6 mm) and then close the chambers. This is the same as having an equalisation port in Solo Air/DebonAir.

    Preferred setup for pretty much all MTB suspension whether air or coil is to have low preload – low force at top out. With DebonAir this goes as far as equilibrium being slightly sucked down and having a pneumatic top out. For Dual Air, this means the negative pressure at topout should be higher than the positive pressure (negative acts against a smaller cross-section area, so this balances out). Pumping until you start to see suckdown is a good approach.

    Shorter travel forks inevitably have to ramp up pretty sharply. This can mean you end up narrowing the operating range by losing a bit at the top and having the sharp ramp up at the bottom. This compromise may be worse than having some preload at top out. All setup is compromise and these are the bounds you’re working within.

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